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UK Amateur Radio Foundation Licence

Syllabus 1.6b

Topic 02 - Technical Aspects (Part 2)

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Resistance and power

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Summary of Prefixes

​

Prefix

Symbol

Scale

Multiplier

x10n

micro

μ (e.g. μA)

Millionth

x0.000001

x10-6

milli

m (e.g. mW)

Thousandth

x0.001

x10-3

None

-

(e.g. A, W, V, Ω, Hz)

-

x1

x100

kilo

k (e.g. kV)

Thousands

x1,000

x103

Mega

M (e.g. MΩ)

Millions

x1,000,000

x106

Giga

G (e.g. GHz)

Billions (US)

x1,000,000,000

x109

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Current, Voltage, Resistance and Power

Current:

  • Unit - Ampere
  • Abbreviation - "A"
  • Symbol - "I"

Voltage:

  • Unit - Volt
  • Abbreviation - "V"
  • Symbol - "V"

Resistance:

  • Unit - Ohm
  • Abbreviation - "Ω"
  • Symbol - "R"

Power:

  • Unit - Watt
  • Abbreviation - "W"
  • Symbol - "P"

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Circuits

All circuits must contain:

  • Source of electrical energy
    • Cell
    • Battery
    • Power Supply
    • Solar Panel
    • Wind Turbine
    • Generator
  • Continuous path between positive (+ve) and negative (-ve)

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Ohms law

V = I × R

I = V ÷ R

R = V ÷ I

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Ohms law example

Determine the current flowing in the circuit below:

​

V = 10V

R = 500Ω

I = ?

By Ohms Law: I = V ÷ R

I = 10 ÷ 500

I = 0.02A = 20mA

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Pop quiz: Ohms law

Determine the resistance of the lamp in the circuit below:

​

V = 1.5V

I = 500mA = 0.5A

R = ?

By Ohms Law: R = V ÷ I

R = 1.5 ÷ 0.5

R = 3Ω

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Joules law

P = I × V

I = P ÷ V

V = P ÷ I

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Joules law example

Determine the power consumed by the Resistor in the circuit below:

​

V = 10V

I = 20mA = 0.02A

P = ?

By Joules Law: P = I × V

P = 0.02 × 10

P = 0.2W = 200mW

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Pop quiz: Joules law

How much power does the lamp in the circuit below use:

​

V = 1.5V

I = 500mA = 0.5A

P = ?

By Joules Law: P = I × V

P = 0.5 × 1.5

P = 0.75W = 750mW

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Frequency

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Frequency to wavelength conversion

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Frequency wavelength conversion

300 = f(MHz) × λ

f(MHz) = 300 ÷ λ

λ = 300 ÷ f(MHz)

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Frequency wavelength conversion examples

What is the frequency of a signal with a wavelength of 15m?

f(MHz) = ?

λ = 15m

f(MHz) = 300 ÷ λ

f(MHz) = 300 ÷ 15

f(MHz) = 20MHz

What is the wavelength of a signal with a frequency of 433MHz?

f(MHz) = 433MHz

λ = ?

λ = 300 ÷ f(MHz)

λ = 300 ÷ 433

λ = 0.693m = 69.3cm ≈ 70cm

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Pop Quiz: Frequency wavelength conversion

What is the frequency of a signal with a wavelength of 30m?

f(MHz) = ?

λ = 30m

f(MHz) = 300 ÷ λ

f(MHz) = 300 ÷ 30

f(MHz) = 10MHz

What is the wavelength of a signal with a frequency of 3.75MHz?

f(MHz) = 3.75MHz

λ = ?

λ = 300 ÷ f(MHz)

λ = 300 ÷ 3.75

λ = 80m

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And finally…

Any questions?